Here is how I would rank them:

1) Predator

2) Predators

3) Predator 2

4) Alien vs Predator

5) Alien vs Predator: Requiem

The original movie is without a doubt the best. Predator 2 isn't as bad as people made it out to be. The Alien vs Predator movies are terrible and should be erased from history. Predators is okay, but largely forgettable and mediocre.

Predator 2

Predators

Predator and Predators are the only good ones, IMO. Predator 2 is passable but it's more of a Danny Glover cop film guest starring Predator.

The original.

Original and and Alien vs Predator. I haven't seen the full Predators though.